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Unused pins on C5505A

john5123
Intellectual 555 points

Can you please advise on the best way to minimise power consumption when the following pins or functions are not used in a C5505A based system?

1) I2C Bus: SDA(B8) and SCL(B9) do not have internal pull-up or pull-down resistors. Should external pull-ups be fitted if the I2C bus is not used?

2) Wakeup (E8): Does this need to be terminated or should it be set as an output and driven low?

3) The EMIF bus: All Pins but D[0..15] and A[15..20] have a specific note to externally terminate the pin if unused.
              3a) What is the best way to make this interface safe?
              3b) Can any termination be achieved without discrete components?
              3c) Can the EMIF D[0..15] pins be left floating or should these be terminated in some manner?

Sorry if this is already documented somewhere, but I couldn't find a suitable reference.


Thanks,
John

  • 0
    TI__Genius 11605 points

     HI John: Please refer to the following documents and let us know if your customer has further questions.

    http://www.ti.com/lit/an/sprab14/sprab14.pdf

    http://www.eetimes.com/design/automotive-design/4004093/Saving-power-with-DSPs-in-portable-apps

     

    Regards,

    Naser

  • 0 in reply to Naser S.
    TI__Genius 11605 points

    Hi John: Let me clarify more please. While these documents do not mention the new C5505A , they can be use as basic reference for power optimization along with the specific device UG and  datasheet.

    Regards,

    Naser.

  • 0 in reply to Naser S.
    TI__Genius 14965 points

    Hi,

    Please refer C5000 Low Power system design wiki covering C5505A http://processors.wiki.ti.com/index.php/C5000_DSP_based_Low-power_system_design

    Regards,

    Hyun

  • 0

    Hi, john5123!

    1) The external pull-ups resistors shouldn't be fitted. They don't play any role if you not use I2C bus.

    2) Wakeup (E8) must be high level = 1. You should connect is to Vcc.

    3)Connect via  resistors.

  • 0 in reply to Naser S.
    Intellectual 555 points

    Hi Naser and Hyun,

    Thanks for your helpful answers to my enquiry. Would you be able to clarify a couple of issues?

    My customer would like to simply tie the SDA and SCL pins to ground, which will stop them from floating without adding any further components. Could this possibly cause any issues for the C5505 DSP, given that the I2C bus isn't being used at all? Any unintended consequences with boot processes, for instance?

    For the EMIF, the plan is to ground all the EMIF signal pins and DVDDEMIF, as described in Table 3-21 of the datasheet. Firmware for the system currently enables the EMIF bus and enables internal pull down resistors on A[15..20]. Is there any danger in leaving this code in place with the EMIF grounded?

    Thanks,
    John

  • 0 in reply to john5123
    TI__Mastermind 30110 points

    Hi all,

    I like this discussion but I think that some experiments should be run before going against the documentation recommendation to use pull-ups even when I2C is not used...

    If SDA and SCL are tied to ground, will the bootloader hang when it attempts to boot from an I2C EEPROM. It should not be an issue if the system boots from a device earlier in the boot sequence... Could quickly test it on the EVM by shorting SDA and SCL to GND, and try a USB boot (check if the USB enumeration occurs in Device Manager). USB boot comes well after I2C in the boot sequence.

    What is the impact on system power? Will it reduce when compared against pull-ups? Cause a temporary spike during the bootloader? I2C does not drive a voltage, but acts as an open collector, so I wouldn't expect much power draw when GND'ed. Could scope the voltage drop (current) across one of the 1ohm resistors to check current spikes..

    I would definitely not tie these pins high because it might sink large current if SDA or SCL is pulled low by the open collector logic.

    I would not leave them floating, because these pins are Hi-Z at reset, and can draw current if not terminated...

    Regards,
    Mark